Tropical Storm Arthur Forecast Discussion

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000
WTNT41 KNHC 172032
TCDAT1

Tropical Storm Arthur Discussion Number   5
NWS National Hurricane Center Miami FL       AL012020
500 PM EDT Sun May 17 2020

Arthur's satellite presentation has not changed much since the
previous advisory.  There are still some fragmented convective 
bands over the eastern portions of the circulation, but convective
activity remains limited over the western half of the storm.  A 
late-arriving ASCAT overpass from around the time of the previous
advisory revealed somewhat lower wind speeds than reported by
this morning's reconnaissance aircraft.  This could be the result 
of the convection becoming more fragmented after the aircraft 
sampled that portion of the storm or related to the low bias of 
the ASCAT instrument.  Regardless, the initial intensity remains a
possibly generous 40 kt for now.  Another reconnaissance aircraft
is scheduled to investigate Arthur this evening.

Arthur is forecast to remain within a low wind shear environment
and over marginally warm sea surface temperatures through early
Monday.  These conditions favor some strengthening, however the
sprawling structure of the cyclone and nearby dry mid-level air are 
likely to temper any increase in wind speed.  After 36-48 hours, 
baroclinic forcing is expected to help the post-tropical cyclone 
maintain its intensity.  Later in the period, the frontal 
gradients decrease which should cause weakening.

Recent satellite fixes show that the Arthur is still moving 
north-northeastward at about 8 kt. The tropical storm should begin 
to accelerate northeastward overnight as a mid-level trough moves 
into the eastern United States.  By Tuesday night, the steering 
flow is expected to become westerly which should cause Arthur to 
turn eastward, then southeastward later in the forecast period.  
The lastest dynamical track guidance has come into a little better 
agreement through 36-48 hours with the GFS and ECMWF converging on 
the previous NHC track. As a result, little change was needed to 
the first couple of days of the earlier NHC track forecast.  After 
that time, most of the guidance has trended toward a faster 
eastward and east-southeastward motion, and the NHC forecast has 
been  adjusted accordingly in that direction as well.


Key Messages:

1. A Tropical Storm Warning is in effect for a portion of the
North Carolina coast.  Tropical-storm-force winds and heavy rains
are expected there on Monday.

2. Dangerous coastal surf conditions and rip currents are expected
to spread northward along the southeast U.S. coast to the
mid-Atlantic states during the next couple of days.  See products
from your local National Weather Service Forecast Office for more
details.


FORECAST POSITIONS AND MAX WINDS

INIT  17/2100Z 31.5N  77.2W   40 KT  45 MPH
 12H  18/0600Z 33.1N  76.4W   40 KT  45 MPH
 24H  18/1800Z 35.3N  74.6W   45 KT  50 MPH
 36H  19/0600Z 36.6N  71.9W   50 KT  60 MPH
 48H  19/1800Z 36.8N  69.2W   50 KT  60 MPH...POST-TROP/EXTRATROP
 60H  20/0600Z 36.3N  67.5W   50 KT  60 MPH...POST-TROP/EXTRATROP
 72H  20/1800Z 35.6N  66.3W   45 KT  50 MPH...POST-TROP/EXTRATROP
 96H  21/1800Z 34.0N  64.0W   35 KT  40 MPH...POST-TROP/EXTRATROP
120H  22/1800Z...DISSIPATED

$$
Forecaster Brown



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